In recent years, a display device including a light-emitting element typified by an EL (Electro Luminescence) element has been developed, and wide utilization is expected by making use of advantages of a self-luminous type such as high image quality, a wide viewing angle, flatness, and lightweight. Since a light-emitting element has characteristics that its luminance is proportional to a current value, there is a display device employing constant current driving by which a constant current flows to the light-emitting element in order to express a gray scale accurately (see Patent Document 1).    [Patent Document 1] Japanese Patent Publication No. 2003-323159